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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

 Different Strokes for Different Folks
~~Generalist vs Specialist Foraging Strategies~~

By Tara Davila
What's in a name?

Herbivores are divided into generalists, animals that eat a wide variety of plants more or less equally, and specialists, animals that rely on one food source, often to the exclusion of all other available foods.  An example of a specialist would be the Koala bear (Phascolarctos vulpecula) of Australia.  They eat Eucalyptus (Eucalyptus spp.) plants only, though there are several species of Eucalyptus available.  Koalas are an example of an extreme specialist, though there are shades of gray involved here as with anything else in nature, meaning that there are specialists that eat other plants at different times of the year, or times of their lives.  An example of one of these specialists would be the pygmy rabbit (Brachylagus idahoensis) found in the Great Basin region of the western U.S.  These critters eat a variety of forbs and grasses in the spring and summer, but during winter exist almost solely on sagebrush (Artemesia spp.) plants.

There are relatively few mammals that specialize on a single plant species as their primary food source, and popular thinking holds that this is because of the high concentrations of plant toxins that would naturally be present in such a diet.  It is thought that exclusive intake of one plant species would exceed the animals' ability to remove or bypass the compounds plants that are toxic to many animals.  It would seem that those that have adapted to be specialists would be very sensitive to changes in their habitat, particularly changes that affect their food.

Interestingly, most specialists eat foods that would be poisonous to other animals.  Many plants contain what are known as anti-quality factors, or allelochemicals, that discourage herbivores from foraging on them.  These anti-quality factors include tannins, terpenoids, alkaloids, and oxalates, and many others.  Tannins and terpenoids are known as quantitative defenses because they act in a dosage dependent fashion, in other words the amount the animal eats determines how sick they will feel.  Specialist herbivores have co-adapted with their preferred food source, they have acquired special traits or organs that allow them to detoxify these allelochemicals.  These can include enlarged livers, enlarged parotid salivary glands, and many others, all of which allow specialists to use foods others cannot.


Tannins bind with proteins in the gastrointestinal tract of herbivores, making them indigestible.  They also tend to taste astringent, which allows animals to recognize and avoid them.  Terpenoids are volatile compounds found in some plants that can cause the microbes in ruminant's stomach to stop growing or die.  These compounds are readily recognizable by their scent, for example if you crush a pine needle in your fingers, the distinctive smell is due to the terpenes.  Some specialists have been shown to use this smell as a method to identify their preferred food source.

The problems with foragers

Herbivores can affect vegetation structure by modifying plant community structure, composition, density, and patchiness.  Toxins found in plants can affect what the herbivores choose to eat.  Both generalist and specialists can affect what plants exist in local stands, but recent studies suggest that they can also affect what plants exist in smaller patches, and even how the plants themselves grow.  While generalist can affect the structure of stands of vegetation through selective foraging, specialists can also exert pressure on specific plants and alter their defensive mechanisms.

Problems between foragers

It has been shown that foraging pressure from novel herbivores can cause plants to shift their chemical composition, increasing or changing the allelochemicals to try and deter the new threat.  Cases in several areas have shown that even when native animals are encouraged to increase population sizes, this increase in pressure can cause the vegetation structure to change, which in turn causes other species to decline.  Yellowstone National Park has recently seen declines in mule deer (Odocoileus hemionus) numbers because elk (Cervus elaphus) are increasing and eating more sagebrush than before.  The sage plants are not able to keep up with the increase in foraging pressure, which has caused a shift in vegetative composition in the region.  Meanwhile mule deer rely on sagebrush for their winter forage, and since the plant is on the decline, so to are the deer.

And then there is us...

In the modern world there has been a lot of movement of plants and animals to areas that are not their traditional homes.  Humans have moved plants to the American and Australian continents from Europe and Asia as food sources.  These introductions of non-native species often cause problems, since we don't also move their control agents, such as predators.  These invasive species are often able to out compete the local species because they have nothing around them that limits their growth, while the native species have all the usual diseases and predators.  Since these introduced plants are often food sources for humans, they are often encouraged to grow as well, though they are often able to migrate away from the farmer's fields and into the wild.


Any person interested in wildlife management needs to take into consideration the habitat requirements, but with specialist herbivores it is critical to think about everything.  These animals are adapted to fill a very narrow niche in the habitat, and when humans or other wildlife cause their one food to change, then the specialist becomes threatened.  As never before we see that ecosystems are interconnected circles, with every aspect having an impact on all others.  We need to consider that this plant that may be inedible and toxic to most animals, may be the key to life for other animals, and anything we do to cause or increase the changes in the vegetation of a region can have serious impacts.  

Suggested readings

Dearing, M. D., A.M. Mangione, and W.H. Karasov.  2000.  Diet breadth of mammalian herbivores: nutrient versus detoxification constraints.  Oecologia.  123:397-405.

Lawler, I. R., W.J. Foley, and B.M. Eschler.  2000.  Foliar concentrations of a single toxin creates habitat patchiness for a marsupial folivore.  Ecology.  81:1327-1338.

Vourc'h, G., J.L. Martin, P. Duncan, J. Escarre, and T.P. Clausen.  2001.  Defensive adaptations to Thuja plicata to ungulate browsing: a comparative study between mainland and island populations.  Oecologia.  126:84-93

Wambolt, C. L.  1998.  Sagebrush and ungulate relationships on Yellowstone's northern range.  Wildlife Society Bulletin.  26:429-437.

Zhang, X., and J.S. States.  1991.  Selective herbivory of ponderosa pine by Abert squirrels: a re-examination of the role of terpenes.  Biochemical systematics and ecology.  19:111-115.

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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/